Saturated and unsaturated chlorinated aliphatic hydrocarbons containing one to three carbon atoms, such as 1,1,2-trichloroethylene (TCE), are commonly used as industrial solvents and their hazards to the environment as well as to the public have been extensively studied. In the past, many industrial companies have improperly and illegally disposed of these chlorinated solvents and as a result such compounds are often found as groundwater contaminants.
One of the most prevalent chlorinated hydrocarbon contaminants in groundwater today is TCE which is widely used industrially for degreasing various metals. Tetrachloroethylene, perchloroethylene (PCE), and dichloromethane, which are widely used for paint stripping and in the dry cleaning industry, are also common groundwater contaminants. Remediation of groundwater containing these low molecular weight compounds is of significant importance and several commercial systems have been recently developed to remove these contaminants not only from the groundwater but also from industrial effluents and waste streams.
Of the systems currently available, above-ground treatment methods have been found to be the most effective for removal of these chlorinated compounds from aqueous effluents. In one such method, highly volatile TCE is stripped from the contaminated water flowing down a column by a countercurrent stream of air. The TCE is adsorbed on granular activated carbon and periodically desorbed from it. The activated carbon is regenerated with steam, and when its efficiency as an adsorbent decreases below about 95% it is replaced. The spent activated carbon adsorbent is eventually buried in a landfill when its adsorptive capacity decreases to a significant extent.
In other above-ground methods, the halogenated organic compounds are destroyed by adding hydrogen peroxide or ozone when an aqueous solution containing the halogenated organic compound is irradiated by high intensity ultraviolet radiation. The use of hydrogen peroxide or ozone for the above purpose is disclosed, for example, in H. Pallet et al., Ozone Science and Engineering, Vol. 9 (1987), pp. 391-418; P. Gehringer et al., Appl. Radiation Isotope, Vol. 43 (1992), pp. 1107-1115; W. D. Sundstrom et al., Hazardous Waste and Hazardous Materials, Vol. 3 (1986), pp. 101-110; and W. H. Glaze et al., Ozone Science and Engineering, Vol. 9 (1987), pp. 335-352.
Recent reports have shown that elemental iron can be employed to dehalogenate many low molecular weight chlorinated hydrocarbons. See, for example, J. E. Barbash et al., American Chemical Society Meeting Abstracts, 1992, April 5-10; L. J. Matheson et al., "Processes Affecting Reductive Dechlorination of Chlorinated Solvents by Zero-Valent Iron", American Chemical Society Meeting Abstracts, 1993 Mar. 28-Apr. 2; L. J. Matheson et al., "Abiotic and Biotic Aspects of Reductive Dechlorination of Chlorinated Solvents by Zero-Valent Iron", American Chemical Society Meeting Abstracts, 1994, March 13-18; and A. Agrawal et al., "Abiotic Remediation of Nitro-Aromatic Groundwater Contaminants by Zero-Valent Iron", American Chemical Society Abstracts, 1994, March 13-18.
These reports show that the degradation of chlorinated hydrocarbons by elemental iron is 10.sup.3 to 10.sup.6 times faster than abiotic or biotic degradation. Batch as well as column experiments show that CCl.sub.4, CHCl.sub.3, TCE and PCE were degraded. Trace quantities of dichloromethylene were found in these investigations, but no vinyl chloride which is an undesirable intermediate in the dechlorination reaction was detected.
At present, an in-ground treatment method using elemental iron is undergoing field investigations. The details of this investigation are published by S. F. O'Hannesin and R. W. Gillham in an article entitled "In-situ Degradation of Halogenated Organics by Permeable Reaction Walls" EPA Ground Water Currents, 1993, pp. 1-2. Specifically, the reference employs a permeable reaction wall that consists of elemental iron and sand. This reaction wall is installed about one meter below the water table in the direction of flow of the plume of contaminated water. Preliminary results of this investigation have indicated that 95% of the TCE and 91% of the PCE in the contaminated water is reduced.
Despite the current interest in using elemental iron for the reductive dechlorination of chlorinated solvents, the initial dechlorination reaction of TCE is too slow and the intermediate reaction products, i.e., 1,1 dichloroethylene and 1,2-dichloroethylene, also dechlorinate very slowly. Given the slow reaction kinetics observed for the dechlorination reaction of TCE, the above-ground remediation of groundwater by a pump and treat method using elemental iron is too slow to be useful for the continuous treatment of large quantities of contaminated water. Thus, continued research is ongoing to develop a rapid and more efficient method for the dechlorination of these halogenated compounds.
New methods for the dechlorination of TCE have been recently developed and reported by S. N. Hassan et al., "Reduction of Halogenated Hydrocarbons with Iron: II. Mechanism of the Reaction", American Chemical Society Abstracts, 1993, Mar. 28-Apr. 2. Specifically, in this abstract, Hassan et al. discloses that the use of nickel- or copper-containing elemental iron may increase or decrease the reaction kinetics of the reductive dechlorination of TCE. The exact details of this investigation, however, have not been published to date.
In order to provide a more efficient method for the reductive dechlorination of TCE from groundwater contaminants, the applicants of the instant invention have conducted extensive research in this area. As a result, the present inventors have found that the use of palladized iron bimetallic system completely and rapidly dechlorinates TCE, as well as many other chlorinated organic compounds, more efficiently than prior art systems.